The invention relates to valves and particularly to a valve seat therein. It will be disclosed, by way of example, in connection with a bidirectional axially pliant pressure assisted seat of a high performance butterfly valve of the type widely used in such diversified applications as chemical processing, power plants, pulp and paper and petrochemical industries. It will readily be appreciated that the valve seat is of general utility, however, and may be used on other types of valves, as for example, ball valves.
Conventional valve seats, as for example, those used on butterfly valves, operate on an interference principle. The disc or closure member interferes with and is forced into the seat member. The closure member is larger than the valve seat so that the two members must be wedged into an interference fit. In a conventional butterfly valve, the seat member is stretched radially to accommodate the larger dimensions of the closure member. However, as the pressures become higher and higher the necessary interference becomes more severe and eventually prevents valve closure.
A number of different approaches have been taken in the prior art to sealingly engage a valve seat with a closure member, all with their attendant disadvantages. One approach is illustrated in U.S. Pat. No. 4,005,848 to Eggleston in which the fluid pressure acting on the valve seat is translated into sliding movement on a conical surface by a metal garter spring. This sliding movement squeezes the seat radially inward against the disc edge. While this seat may be effective in some applications, certain problems with the metal garter spring have been encountered, especially in corrosive applications where the metal garter spring corroded, resulting in leakage across the close valve. Also, under applications with prolonged high pressure, the thin unsupported web or leg of the seat tends to cold flow, that is, take a permanent set or change in shape or dimension when subjected to prolonged stresses. The web is then no longer able to force the seat radially inward against the disc. As a consequence leakage occurs across the closed valve.
Another approach, disclosed in U.S. Pat. No. 3,734,457 to Roos, has been to provide a valve seat with an inside diameter smaller than the disc. In this approach the seat is restrained against radial stretching by a metal hoop, and an interference fit is achieved between the seat and the disc. When the disc is closed, it compresses the seat and at the same time stretches the metal hoop radially.
While this metal hoop approach has been effective in some applications, certain problems have been encountered. For example, in high pressure applications the metal hoop will be subjected to excessive radial stretching due to movement of the disc resulting in permanent deformation of the hoop with subsequent leakage across the closed valve. In addition, since the metal hoop is exposed to media flowing through the valve, it will corrode when used in certain hostile environments. This corrosion results in a seat which is not able to achieve the radial inward compression necessary to effect leak-tight shut-off between the seat and the disc.
Still another prior art approach is described in U.S. Pat. No. 3,563,510, where the valve seat is retained in an annular recess in the valve body. The seat moves into the corner of the recess under the influence of fluid pressure which produces radial inward forces against the disc. This effect is much the same as a standard O-ring would act under pressure. The seat, comprised of a plastic hollow envelopment containing an elastomeric O-ring has proven effective in some applications. Nevertheless, certain problems have been encountered with this approach. The elastomeric O-ring, the primary sealing member of the seat, is subject to chemical attack and swelling by certain chemicals that permeate the plastic hollow envelope causing the inside diameter of the seat to become considerably smaller than the disc, thereby prohibiting the valve from closing fully. Moreover, even when it is possible to close the disc, permanent damage to the seat may result with attendant leakage. Further, in steam service or other elevated temperature service, the elastomeric O-ring will lose elasticity and allow the hollow envelope to deform permanently resulting in leakage across the closed valve.
Still another prior art approach is disclosed in U.S. Pat. No. 3,608,861 and in Canadian Pat. No. 695,037. This last mentioned approach provides a valve seat retained in a valve body recess and rigid annular projection on the valve body side of the annular groove, the projection functioning as a fulcrum to limit flexure under the influence of high pressure to improve valve shut off. While this seat design has also been successful in some applications, its inherent deficiencies have resulted in problems when used in applications where pressure is applied to both sides of the valve. The effectiveness of the valve seat has been limited by its inability to shut off against high pressures entering the valve body side of the valve. Since the disc movement in butterfly valves is not isotropic, the disc is forced further into the seat when the valve is pressurized in the opposite direction. This arrangement causes the all TEFLON seat which is utilized, with its inherent limited elastic memory, to cold flow and permanently deform radially outward, causing leakage across the close valve. Also, the totally unsupported seal depends entirely on the physical strength of the TEFLON material from which it is made. Fillers, such as glass and carbon have been used in an attempt to strengthen the seat. This type of seat strengthening, however, makes the seat very abrasive, creating rapid wear on the disc sealing surface and eventual leaking across the closed valve. The geometry of the solid TEFLON seat is such that under high pressure applications, above 700 p.s.i., the seat cannot follow the disc movement effectively without cold flowing and losing the sealing capability.
Accordingly, it is an object of the present invention to provide an improved valve seat which effectively seals differential fluid pressure in either direction and which compensates axially rather than radially for the movement of the valve disc.
It is another object of the present invention to provide a valve seat with an axially pliant supporting member embedded therein to provide restorative forces whenever the valve seat is flexed axially.
It is a further object of the present invention to provide an effective seal over a wide temperature range for corrosive and noncorrosive media.
It is a further object to provide a valve with extended cycle life and compensation for the effects of thermal expansion or contraction as well as wear of the seat material.
It is still a further object to provide a valve seat with a total encapsulated pliant membrane that is not subject to corrosive, chemical or elevated temperature attack.
A further object of the invention is to provide a valve seat with a relatively narrow central waist section to facilitate axial flexing.
It is yet another object of the present invention to provide a valve seat that is inherently axially pliant and better able to follow anti-isotropic movements of a valve disc.
It is still another object of the present invention to provide a valve seat with an integral pliant membrane which is protected against seal blow-out under high differential pressures and fluid velocities by a wide retaining flange which is substantially larger than its central narrow waist section.
It is still another object of the present invention to provide a valve seat whose axially pliant movement is limited by first and second side walls of an annular groove.
It is still another object of the present invention to provide a valve seat which is symmetrical about a central axis and easily replaced in the field without the possibility of being installed backwards.
Additionally, another object of the present invention is to provide a valve with a movable seat and reinforcing assembly fitted into a groove of the valve capable of aligning itself with other elements of the valve during assembly.
These and other objects and advantages of the invention will be more readily apparent upon reading the following detailed description and upon reference to the accompanying drawings.